Tumor-induced Osteomalacia Due to Phosphaturic Mesenchymal Tumor, Mixed Cell Type, of the Sphenoid Bone

ABSTRACT Objective: Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by small endocrine tumors that secrete fibroblast growth factor 23 (FGF23), a phosphaturic hormone. Abbreviations: CT = computed tomography; FGF23 = fibroblast growth factor 23; Ga68-DOTANOC = (68)Galliumlabeled (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)-1-NaI(3)-octreotide; MRI = magnetic resonance imaging; PET = positron emission tomography; PMTMCT = primitive appearing mixed connective tissue tumors; SSR = somatostatin receptor; TIO = tumor-induced osteomalacia Methods: A 44-year-old male was evaluated following complaints of progressive leg pain, difficulty walking, and muscle pain over the previous 9 years. Biochemical evaluation showed low serum phosphorus, high urine phosphorus, and elevated FGF23 levels. These findings were suggestive of TIO. A whole-body (68)Gallium-labeled (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)-1-NaI(3)-octreotide positron emission tomography–computed tomography scan revealed a large vascular lesion in the anterior squamous portion of the right temporal lobe and right greater wing of the sphenoid bone, involving the posterolateral wall of the right orbit. Axial T1-weighted magnetic resonance imaging revealed isointense lesions containing multiple vascular channels. The histomorphologic feature of the excised tumor was compatible with a phosphaturic mesenchymal tumor, mixed cell type. Serum phosphorus and FGF23 levels normalized after excision of the tumor and resulted in healing of the fractured femur. Results: We report a large vascular lesion in the anterior squamous portion of the right temporal lobe and right greater wing of the sphenoid bone, involving the posterolateral wall of the right orbit as a cause of TIO. Conclusion: TIO is a fascinating paraneoplastic syndrome and is an important cause of adult-onset hypophosphatemia. Localization of tumors in cases of TIO is difficult and a stepwise approach with functional and anatomic imaging is usually successful in 90% of the cases. Excision of the tumor cures this debilitating disease, although recurrence is possible.